Capacitive micromachined ultrasonic transducers (CMUTs) are gaining increasing popularity in the fields of medical and underwater imaging. In addition, CMUT technology has recently been used for applications such as high intensity focused ultrasound (HIFU) therapy and resonating chemical sensors. The basic structure of a CMUT includes a thin membrane and a support substrate separated by a vacuum cavity. Typically, a doped silicon substrate makes up the bottom electrode of the capacitor and a conducting membrane acts as the top electrode. The membrane vibrates when excited with an electrical AC signal. Conversely, an electrical signal is generated when the membrane vibrates due to impinging sound waves.
CMUTs were originally fabricated using a sacrificial release process. In this process, a silicon nitride membrane layer is deposited on a patterned sacrificial polysilicon layer; the polysilicon is subsequently removed via small channels; and then the resulting gap is vacuum sealed by a second silicon nitride layer deposited on top of the membrane; the final membrane thickness is set by etching back the second nitride layer. This technique has numerous intrinsic drawbacks, including: stiction problems that may prevent the release of the membrane; stress in the membrane that is very sensitive to deposition conditions; difficulties in controlling the membrane thickness due to successive deposition and etching steps; and difficulties to control the gap height or thickness due to the unwanted non-uniform nitride deposition in the cavity during sealing.
More recently, CMUT fabrication processes were developed utilizing a direct wafer bonding (fusion bonding) technique. In this technique, the vacuum cavities are formed by etching an oxide layer before the wafer is bonded to a silicon-on-insulator (SOI) wafer in a vacuum chamber. After removing the handle wafer and the buried oxide (BOX) layer of the SOI wafer, a single crystal silicon layer remains as the CMUT membrane with good uniformity and without significant residual stress. However, since the gap height is determined through an etching process, gap height control is difficult. In addition, the minimum gap height is limited by the thickness of the original oxide layer, requiring design compromise in terms of breakdown voltage and parasitic capacitance.
The present invention addresses at least the difficult problems of fabricating CMUTs and advances the art with a method of fabricating a CMUT using local oxidation.